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A fully coupled fluid–structure piezoelectric model is presented based on the finite element method that is capable of modeling piezoelectric harvesters in the presence of free-surface flow and floating lightweight harvesters with arbitrary movements. The Navier–Stokes equations and the phase-field method are employed to describe the free-surface waves. Equations of the conservation of linear momentum in company with the piezoelectric constitutive relations in the strain-charge form are utilized to obtain solid deformation and the electric field intensity. According to the results, attaching mass to the tip of the beam leads to a 13.5% rise in the output voltage compared to the state without the attached mass. Another studied factor was the influence of load resistance on voltage and the output power. The generated voltage grows along with the load resistance until it reaches a constant value. However, the power has an optimum load resistance of 2.61 times higher than the reference state. The beam’s inclination is significant in effectively exploiting water waves due to raising the root mean square (RMS) value of the voltage by 89.53% at an angle of 40 degrees relative to the vertical state. By altering the thickness of the beam from 1 mm to the value of 1.5 mm, the RMS voltage exhibits a considerable upward change of 66%. By increasing the length of the cantilever beam connected to the buoyant structure, and therefore, the indentation of the beam in the water, the output voltage grows, such that a beam with the length of 25 cm shows a 2.92 times increase in the output voltage relative to the beam with a length of 15 cm.